Method for coordinating a vehicle dynamics control system with an active normal force adjustment system

ABSTRACT

A vehicle dynamics control system for a motor vehicle having an active normal force adjustment system with which the normal force acting on a wheel may be adjusted is described. For coordinating the vehicle dynamics control system with the active normal force adjustment system, information about a change in the normal force is to be supplied to a control unit of the vehicle dynamics control system and may be taken into account in the vehicle dynamic control.

FIELD OF THE INVENTION

[0001] The present invention relates to a vehicle dynamics controlsystem for a vehicle having an active normal force adjustment system,and a method for coordinating a vehicle dynamics control system with anactive normal force adjustment system.

BACKGROUND INFORMATION

[0002] Vehicle dynamics control systems, understood to include all thedevices that intervene in the driving operation via operation of brakesor drive such as ABS (anti-lock brake system), TCS (traction controlsystem), ESP (electronic stability program) or MSR (engine drag torquecontrol), help to stabilize motor vehicles in borderline situations inparticular. To further improve controllability, vehicles areincreasingly being equipped with active normal force adjustment systems,which are also referred to as spring-damper systems with which thenormal force (wheel contact force) of a wheel is adjustable as afunction of the driving situation. The function of the active normalforce adjustment system such as CDC (continuous damper control) or ARC(active roll control) is to reduce vertical acceleration of the vehiclebody and/or to compensate for the rolling motion of the vehicle whenturning a corner and in horizontal leveling of the vehicle.

[0003] A two-channel ARC system has, for example, actuators on the frontand rear axles which may be under tension independently of one anotherwith respect to the passive state. However, when there is differenttension on the front and rear axles, the normal forces (contact forces)on the wheels change. Because the lateral guidance force of the wheelsincreases only degressively as the normal force increases, theself-steering effect of the vehicle thus also changes. Depending on thesetting of the normal force adjustment system, the vehicle thus exhibitseither a more oversteering behavior or a more understeering behavior incomparison with the passive state. This has negative effects inparticular on a vehicle dynamics control which is performed in parallel.

[0004] Vehicle dynamics control systems are usually based on a fixedself-steering effect. A self-steering effect altered by the normal forceadjustment system may therefore result in faulty braking operations whenthe actual performance of the vehicle differs too much from thecalculated setpoint performance.

[0005] In addition, the traction control implemented as part of thedynamics control is impaired. In order to adapt the brake slipcontroller to the particular driving situation, the normal forces actingon the wheels are usually estimated. Operation of the normal forceadjustment system results in a deviation between the estimated andactual normal forces and may thus result in a malfunction of thetraction control system.

SUMMARY

[0006] An object of the present invention is to create a method withwhich a vehicle dynamics control may be coordinated with a normal forceadjustment system and to create a suitably adjusted vehicle dynamicscontrol system.

[0007] In accordance with an example embodiment of the presentinvention, the vehicle dynamics control system is supplied withinformation about the change in at least one wheel normal force duringoperation of the normal force adjustment system, so that thisinformation may be taken into account by the vehicle dynamics controlsystem in performing its regulation. A vehicle dynamics control systemand a normal force adjustment system may be coordinated optimally inthis way, and faulty braking operation in particular on the part of thevehicle dynamics control system may be prevented.

[0008] The information about the change in the normal forces may be anyinformation from which a change in normal forces, e.g., a change value,the absolute wheel contact force, etc., may be determined.

[0009] According to a first embodiment of the present invention, theinformation about the change in normal force is used to correctestimated normal forces. As part of a vehicle dynamics control, thenormal forces acting on a wheel are usually estimated by a mathematicalalgorithm, e.g., from the transverse acceleration and the longitudinalacceleration of the vehicle. The estimated normal force values arepreferably corrected by modifying the normal forces during operation ofthe normal force adjustment system. This yields the normal forcesactually in effect, and then traction control, for example, may beperformed on the basis of these values.

[0010] According to another embodiment of the present invention, theinformation about the change in normal forces is used to calculate oneor more setpoint values for the transverse motion and yawing motion ofthe vehicle.

[0011] In the case of a vehicle dynamics control system having yaw rateregulation, a setpoint yaw rate is usually calculated, depending on acharacteristic velocity, which in turn depends on the self-steeringeffect of the vehicle. The setpoint yaw rate is usually calculated usingthe Ackermann equation, which is also known by the term “single-trackmodel.” Using information about the change in normal force supplied bythe normal force adjustment system, the characteristic velocity and thusthe setpoint yaw rate may be adjusted accordingly.

[0012] In the case of a vehicle dynamics control system having floatangle regulation, a setpoint value for the float angle is alsodetermined in addition to or as an alternative to the setpoint yaw rate.The setpoint float angle may also be calculated using the single-trackmodel. The parameters for calculating the setpoint float angle may beadjusted accordingly using the information about the change in normalforce supplied by the normal force adjustment system.

[0013] According to another embodiment of the present invention, in thecase of an oversteering adjustment of the normal force adjustmentsystem—i.e., the vehicle exhibits oversteering behavior—an interventionthreshold of the vehicle dynamics control is raised. A controlintervention by the vehicle dynamics control is performed in this caseonly when there is a great system deviation in the controlled variable(e.g., yaw rate or float angle) so that unwanted brake operations inparticular may be avoided.

[0014] The information about the change in normal force supplied by thenormal force adjustment system is preferably monitored for plausibility.This makes it possible to prevent a wrong adjustment of the vehicledynamics control system.

[0015] The present invention is explained in greater detail below on thebasis of the accompanying Figures as examples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 shows a control system, including a vehicle dynamicscontrol system and a normal force adjustment system.

[0017]FIGS. 2a and 2 b show a flow chart to illustrate the essentialmethod steps in coordinating a vehicle dynamics control system with anormal force adjustment system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0018]FIG. 1 shows a schematic diagram of the system architecture of acomplex control system, which includes a vehicle dynamics control systemhaving components 1, 3, 4, 6 and a normal force adjustment system havingcomponents 2, 5, 8.

[0019] Vehicle dynamics control system 1, 3, 4, 6 includes a controlunit 1 in which a control algorithm (ESP in the present case) is storedas a program module, a sensor system 3 for determining the controllerinput variables (actual performance) and multiple final controllingelements 4, 6 such as, for example, an engine control unit, a steeringcontroller, etc., and a wheel brake 6 for influencing the drivingperformance. When a predetermined intervention threshold is exceeded,i.e., a predetermined system deviation of the vehicle occurs, brake 6,for example, is operated to adapt the yaw performance of the vehicle tothe setpoint and to thus stabilize the vehicle.

[0020] Normal force adjustment system 2, 5, 8 includes a second controlunit 2 connected to a sensor system 5 for measuring various statevariables and an actuator 8 (active spring-damper element).(Alternatively the vehicle dynamics control algorithm and the normalforce adjustment algorithm may also be implemented in a single controlunit. The interface is then inside the control unit.) Actuator 8 may beoperated by control unit 2 to change the normal force of a wheel 7. Thisis used in particular to reduce the vertical acceleration of the vehiclebody, to compensate for a rolling motion of the vehicle when turning acorner and/or for horizontal leveling of the vehicle.

[0021] Vehicle dynamics control system 1, 3, 4, 6 determines normalforces F_(N) acting on wheels 7 for implementing a yaw rate regulation,for example. These forces are usually estimated on the basis of thetransverse and longitudinal acceleration of the vehicle, theacceleration values being input by sensors or determined by anestimation method. When there is a change in the normal forces due tooperation of normal force adjustment system 2, 5, 8, vehicle dynamicscontrol system 1, 3, 4, 6 must be adjusted accordingly.

[0022] Changes in normal force ΔF_(N,XY) (XY=left front (VL), rightfront (VR), left rear (HL), right rear (HR)), which are provided bynormal force adjustment system 2, 5, 8, are supplied to ESP control unit1 for this purpose.

[0023] In the case of a normal force adjustment system 2, 5, 8 havingonly one actuator 8 per axle, it is sufficient if only one signal forthe change in normal force ΔF_(N,XY) is transmitted to control unit 1for each axle, because changes in normal force ΔF_(N,XY) on the left andthe right have the same absolute value but different signs. In this caseit holds that:

ΔF _(N,VL) =−ΔF _(N,VR) ;ΔF _(N,HL) =−ΔF _(N,HR)  (1)

[0024] Optionally some other information, which may yield the changes innormal force ΔF_(N,XY), could be transmitted to ESP control unit 1.

[0025] Normal force signals ΔF_(N,XY) transmitted are preferablymonitored for plausibility. This makes it possible to prevent incorrectadaptation of vehicle dynamics control system 1, 3, 4, 8 when there arefaulty signals or a faulty transmission.

[0026] For monitoring normal force change signals ΔF_(N), standard testssuch as time-out monitoring, exceeding an admissible range, or exceedinga maximum change gradient could be performed.

[0027] In addition, long-term monitoring is preferably performed on thebasis of the sum of the changes in normal force over all wheels 7. Sincethe sum of normal forces F_(N,XY) is equal to the force due to weight ofthe vehicle when averaged over time, the sum of changes in normal forceΔF_(N,XY) must be equal to zero when averaged over time. It thus holdsthat:

ΔF _(N,sum) =ΔF _(N,VL) +ΔF _(N,VR) +ΔF _(N,HL) +F _(N,HR)=0  (2)

[0028] Summation signal ΔF_(N,sum) is preferably low-pass filtered:

ΔF _(N,sum,filt) =TP{ΔF _(N,sum)}  (3)

[0029] A fault is recognized as soon as the filtered signal exceeds apredetermined threshold C₁ in absolute value, where the following holds:

|ΔF _(N,sum,filt) |>C ₁  (4)

[0030] In a normal force adjustment system 2, 5, 8 having only twoactuators on the front and rear axles, long-term monitoring cannot beperformed because the condition is satisfied here generically. Theinformation provided by normal force adjustment system 2, 5, 8 about thechange in normal force ΔF_(N,XY) may be taken into account by vehicledynamics control system 1, 3, 4, 6 in different ways:

[0031] The information may first be used to correct normal forces F⁰_(N,XY) estimated by control unit 1:

F _(N,XY) =F _(N,XY) +ΔF _(N,XY)  (5)

[0032] Corrected normal forces F_(N,XY) are made available to the brakeslip controller in particular.

[0033] Secondly, the information may be used to correct the computationof a setpoint yaw rate during a yaw rate control.

[0034] The setpoint yaw rate has been calculated from vehicle velocity vand wheel steering angle δ_(v) as variables, as well as wheelbase L andself-steering parameter v_(ch) (characteristic velocity) as constantapplication parameters. As a rule, this is carried out by using theAckermann equation, also known as the “single-track model”:$\begin{matrix}{{{\Psi_{setpoint}}/{t}} = {{\frac{1}{L} \cdot \frac{v}{1 + \left( {v/v_{ch}} \right)^{2}} \cdot \tan}\quad \delta_{v}}} & (6)\end{matrix}$

[0035] To take the altered self-steering effect of the vehicle intoaccount, changes in normal force ΔF_(N,XY) and transverse accelerationa_(y) enter into the computation of setpoint yaw rate dψ_(setpoint)/dtwhere it holds that:

dΨ _(setpoint) /dt=f(V,δ _(v) ,a _(y) ,ΔF _(N,XY))  (7)

[0036] Characteristic velocity v_(ch) is regarded here as a variablewhich depends on changes in normal force ΔF_(N,XY):

v _(ch) =f(a _(y) ,ΔF _(N,XY))  (8)

[0037] The self-steering effect depends on the difference between thedisplacements of normal force acing on the front axle and the rear axle:

ΔF _(d)=(ΔF _(N,VL) ,−ΔF _(N,VR))−(ΔF _(N,HL) −ΔF _(N,HR))  (9)

[0038] Thus the following holds for characteristic velocity v_(ch):

v _(ch) =v _(ch) ⁰*(1−K*a _(y) *ΔF _(d))  (10)

[0039] Application parameter v_(ch) ⁰ is a constant (without normalforce intervention) and K is an influencing factor in the case of anormal force intervention.

[0040] To determine parameters v_(ch) ⁰ and K, for example, driving in acircle may be performed under predefined conditions. Normal forceadjustment system 2, 5, 8 is adjusted to different values of ΔF_(d) in atest series. Factor K and/or characteristic curve

(see equation 11) may thus be determined on the basis of measurements ofsteering angle δ_(v) at the wheel, driving speed v, yaw rate dψ/dt, andtransverse acceleration a_(y).

[0041] Instead of the linear approximation equation according toequation 10, the characteristic velocity may also be represented asfunction

whose interpolation points are given by the product a_(y)*ΔF_(d):

v _(ch) =v _(ch) ⁰*γ(a _(y) *ΔF _(d))  (11)

[0042] The vehicle may also be adjusted to oversteer by a correspondingadjustment of normal force adjustment system 2, 5, 8. In this range, theAckermann equation (6) is no longer valid because it only describes theyaw rate in understeering behavior. To nevertheless obtain useful valuesfor the setpoint yaw rate, a very high value is selected forcharacteristic velocity v_(ch). This describes an approximately neutralself-steering effect. In addition, the intervention threshold of the yawrate controller is preferably widened, i.e., regulation is performedonly at a greater system deviation. The intervention threshold of theyaw rate controller is preferably a function of changes in normal forceΔF_(N,XY):

dΨ _(threshold) /dt=f(a _(y) ,ΔF _(N,XY))  (12)

[0043] By analogy with equation 11, this may also be represented in theform of a characteristic curve T:

dΨ _(threshold) /dt=T(a _(y) *ΔF _(D))  (13)

[0044] Normal force adjustment system 2, 5, 8 may also be triggered byvehicle dynamics control system 1, 3, 4, 6 to adjust the normal forcedistribution in the desired manner. Vehicle dynamics control system 1,3, 4, 6 may request normal force adjustment system 2, 5, 8 to set, forexample, a neutral position via a suitable signal. Control unit 1transmits a signal Def to normal force adjustment system control unit 2for this purpose.

[0045] During transmission of a faulty normal force change signalΔF_(N,XY), the normal force change values ΔF_(N) that are input arepreferably not taken into account by vehicle dynamics control system 1.In this case regulation is performed on the basis of preset values, forexample. Normal force adjustment system 2, 5, 8 is also requested toswitch to a passive state, i.e., the changes in normal force are reducedto zero.

[0046]FIGS. 2a through 2 c show a flow chart depicting the essentialmethod steps in coordinating a vehicle dynamics control system with anormal force adjustment system 2, 5, 8. The method steps known from therelated art are depicted in the form of non-hatched blocks and the newlyadded method steps are depicted in the form of hatched blocks.

[0047] In a first method step 10, the sensor signals of sensor system 3are first input by ESP control unit 1 and in step 11 they are monitoredand conditioned. In step 12, normal forces ΔF⁰ _(N,XY) are estimated. Instep 13 changes in normal force ΔF_(N,XY) which are supplied by controlunit 2 of normal force adjustment system 2, 5, 8 are then input into ESPcontrol unit 1 and in step 14 they are monitored for plausibility.

[0048] A check is performed in step 15 to determine whether the changesin normal force are plausible (yes) or not (no). If yes, estimatednormal forces F⁰ _(N,XY) are corrected by changes in normal forceΔF_(N,XY) (step 17). If the changes in normal force ΔF_(N,XY) that havebeen input are not plausible, ESP control unit 1 outputs a passivationrequest to control unit 2 (step 16) causing normal force adjustmentsystem 2, 5, 8 to switch to a normal position.

[0049] In the case of plausible values ΔF_(N,XY) characteristic velocityv_(ch) is also corrected in step 18 according to equation 10 or 11.

[0050] In step 19 a setpoint yaw rate dψ_(setpoint)/dt is calculatedaccording to equation 6. In addition, a check is performed in step 20 todetermine whether normal force adjustment system 2, 5, 8 is set tooversteer. If the answer is yes, then in step 21 the control thresholdsare adjusted as a function of the normal forces. If the answer is no,there is no adjustment of the control thresholds.

[0051] The yaw rate controller contained in control unit 1 is updatedaccordingly in step 22.

[0052] A check is performed in step 23 to determine whether or not themotion of the vehicle is oversteering. If the answer is yes, then instep 24 an oversteering warning is output to control unit 2 of normalforce adjustment system 2, 5, 8, causing normal force adjustment system2, 5, 8 to perform a neutral or understeering adjustment. If the answeris no, no oversteering warning is output. Finally, in step 25, the brakeslip controller and other controllers receive the particular setpointvalues from the yaw rate controller.

List of Reference Notation

[0053]1 ESP control unit

[0054]2 normal force adjustment system control unit

[0055]3 ESP sensor system

[0056]4 actuators

[0057]5 normal force adjustment system sensors

[0058]6 wheel brake

[0059]7 wheel

[0060]8 actuator

[0061]10-25 method steps

[0062] ΔF_(N) change in normal force

What is claimed is:
 1. A vehicle dynamics control system for a vehicle,comprising: an active normal force adjustment system for changing anormal force acting on a wheel; and a vehicle dynamics control system incommunication with the active normal force adjustment system, whereinthe active normal force adjustment system is configured to supplyinformation about a change in the normal force to the vehicle dynamiccontrol system, the vehicle dynamics control system configured to takethe supplied information into account in regulating the vehicle.
 2. Thevehicle dynamics control system as recited in claim 1, wherein thenormal forces acting on the wheel are estimated by using a mathematicalmodel, and the estimated normal forces are corrected based on theinformation supplied about the changes in normal force.
 3. The vehicledynamics control system as recited in claim 1, wherein the informationabout the change in normal force is taken into account in calculating asetpoint quantity.
 4. The vehicle dynamics control system as recited inclaim 1, wherein a characteristic velocity is determined as a functionof the information about the change in the normal force.
 5. The vehicledynamics control system as recited in claim 1, wherein an interventionthreshold of the vehicle dynamics control is adjusted as a function ofthe information about the change in the normal force.
 6. The vehicledynamics control system as recited in claim 1, wherein the vehicledynamics control system monitors the information about the change innormal force for plausibility.
 7. The vehicle dynamics control system asrecited in claim 1, wherein the vehicle dynamics control system isconfigured so that the active normal force adjustment system istriggerable.
 8. A method for coordinating a vehicle dynamics controlsystem with an active normal force adjustment system for changing anormal force acting on a wheel comprising: providing information about achange in the normal force; transmitting the information to the vehicledynamics control system; and taking the information into account in thevehicle dynamic control.
 9. The method as recited in claim 8, wherein inthe case of a vehicle set to be oversteered by the active normal forceadjustment system, an intervention threshold of the vehicle dynamicscontrol is raised.